Isobutylene-isoprene polymers, generally termed “butyl rubbers”, have been well known since the 1930s and their synthesis and properties are described by Kresge and Wang in 8 KIRK-OTHMER ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY 934-955 (4th ed. 1993). These butyl rubber polymers have good impermeability to air and a high level of damping when stretched or compressed, and are used extensively throughout the tire and pharmaceutical industries. The copolymers are made by a cationic slurry polymerization process at approximately −95° C. using a catalyst comprising a Lewis Acid and an initiator. Initiators such as water and anhydrous HCl are used extensively. Related patents are EP 0 279 456; WO 00/40624; U.S. Pat. Nos. 4,385,560, 5,169,914, and 5,506,316, herein incorporated by reference.
The commercial reactors used to make these rubbers are well mixed vessels of greater than 10 to 30 liters in volume with a high circulation rate provided by a pump impeller. The polymerization and the pump both generate heat and, in order to keep the slurry cold, the reactor contains a heat exchanger. One embodiment of such a continuous flow stirred tank reactor (“CFSTR”) is found in U.S. Pat. No. 5,417,930, incorporated by reference, hereinafter referred to in general as a “reactor” or “butyl reactor”. In these reactors, slurry (reacted monomers) is circulated through tubes of a heat exchanger by a pump, while boiling ethylene on the shell side provides cooling, the slurry temperature being determined by the boiling ethylene temperature, the required heat flux and the overall resistance to heat transfer. On the slurry side, the heat exchanger surfaces progressively foul, often referred to as film fouling, which causes the slurry temperature to rise. This often limits the practical slurry concentration that can be used in most reactors from 21 to 28 wt % relative to the total weight of the slurry, diluent, and unreacted monomers.
As the slurry temperature increases, there is evidence that the slurry viscosity rises, causing a measurable reduction in the heat transfer coefficient and a further increase in slurry temperature. The increase in temperature will cause a further increase in viscosity and the progression continues until the slurry becomes unstable and starts to agglomerate which can lead to reactor plugging. Consequently, reactors experiencing rapid warm up, often referred to as run away, are taken out of service quickly to avoid fouling and plugging, and subsequent plant upsets.
Reactor “warm-up” then refers to the gradual rise in the temperature of the reactor as a polymerization run progresses. At a constant polymerization rate, the warm-up is the result of a progressive loss of heat removal capability in the reactor. The heat removed from the reactor can be represented mathematically by the following equation (1).Q=(U)(A)(Tslurry−Tethylene)  (1)where “Q” is the heat removed, “A” is the surface area of the reactor, “U” is the overall heat transfer coefficient, which is a composite of several heat transfer coefficients for the slurry itself, the walls of the reactor, the film formed on the reactor wall, and the boiling ethylene used to draw heat from the exothermic polymerization reaction. The “T” values are the temperatures of the slurry and ethylene, respectively.
In the polymerization process, the temperature difference driving force for heat transfer must increase if (a) the overall heat transfer coefficient U decreases, and/or (b) the heat transfer area is lost during a reactor run, such as by plugged tubes. Both can occur as a result of film formation and mass fouling of the reactor. Also, U will decrease if the reactor circulation rate drops or the slurry viscosity increases. Although not wishing to be bound by the following mathematical relationship, the slurry side heat transfer coefficient can be related to the viscosity of the slurry by the Sieder-Tate equation for turbulent fluid flow as shown below in equation (2):                                                         h              slurry                        ⁢            D                    k                =                              (            0.023            )                    ⁢                                    (                                                D                  ⁢                                                                           ⁢                  v                  ⁢                                                                           ⁢                  ρ                                                  μ                  b                                            )                        0.8                    ⁢                                    (                                                                    μ                    b                                    ⁢                                      c                    p                                                  k                            )                        0.4                    ⁢                                    (                                                μ                  b                                                  μ                  w                                            )                        0.167                                              (        2        )            where hslurry is the slurry side heat transfer coefficient, D is the diameter of the reactor heat transfer tube, k is the thermal conductivity of the reactor polymerizing slurry, ν is the average velocity of the slurry inside the tube, ρ is the average density of the slurry, μb is the average bulk viscosity of the polymerizing slurry, cp is the specific heat of the polymerizing slurry, and μw is the average wall viscosity of the polymerizing slurry. Therefore, hslurry is proportional to (1/μb)0.4 in equation (2).
Operating problems associated with using these reactors vary depending upon the specific reaction taking place and the specific location within the reactor. One problem with these reactors is the presence of non-homogenous zones beneath (or above) the pump impeller where feed is introduced. The monomer-rich zone adjacent the pump can be particularly troublesome because feed may be introduced with as high as 40% monomer concentration, whereas the steady-state monomer level in the reactor is much lower, typically from 1% to 10%. The inventors have found that, surprisingly, if an initiator such as a C5 or greater tertiary halo-alkyl is added to the system, the reactor heat transfer efficiency improves, consistent with a reduction in viscosity of the slurry. This is unexpected for at least two reasons.
First, the use of the initiator 2-chloro-2,4,4-trimethyl-pentane (TMPCl) has been demonstrated in the polymerization of an olefin and the highly reactive para-alkylstyrene, as disclosed in U.S. Ser. No. 09/684,713, filed on Oct. 6, 2000 (assigned to the assignee of the present application). However, conjugated dienes, such as used in butyl rubber production, are known to act as retarding monomers in polymerizations. This observation would tend to teach away from using a TMPCl or other C5 or larger initiators in the polymerization of butyl rubber.
Second, certain tertiary alkyl halide initiators such as tert-butylchloride (a C4 tertiary halide) have been shown by Kennedy et al. in U.S. Pat. No. 3,560,458 to improve isobutylene polymerization in small scale, batch experiments when compared to HCl. Yet, there is little to no improvement when comparing tert-butylchloride and TMPCl in small scale batch experiments. Further, the lack of steady state conditions in the small batch process means that heat transfer and viscosity changes would not be apparent when going to a continuous, slurry process, nor would the associated problem of reactor fouling.
The inventors have unexpectedly found that certain alkyl halide compounds greater than C4 significantly reduces reactor fouling associated with using HCl as an initiator for butyl rubber polymerization in continuous slurry reactors. The present invention enables a higher slurry concentration and/or longer run lengths than would otherwise be practical in most commercial reactors.